The simplest form of x-ray imaging equipment utilizes projection imaging in which an object to be imaged is placed between an x-ray source and a two-dimensional x-ray recorder. Portions of the object that absorb or scatter x-rays are seen as a shadow on the image formed by the recorder. Unfortunately, these devices do not provide any information on the three-dimensional structure of the object. If the object contains a region of dense, x-ray absorbing material, the resultant “shadow” makes it difficult to see any other features that are between the x-ray source and the recorder for which the x-rays pass through the absorbing material.
This shortcoming of projection imaging is overcome by CT (computed tomography) scanners. CT scanners combine information from a variety of projection viewpoints to overcome the shadowing and generate a 3D (three-dimensional) description of the object. A typical CT system measures the x-ray flux reaching a detector from a source that moves around the object. The object being scanned is modeled by a plurality of voxels having unknown x-ray absorbency. At each point, the measured flux represents the weighted sum of the x-ray absorbencies of each voxel along the path from the x-ray source to the detector. Different paths provide weighted sums involving different sets of voxels. If sufficient points are measured, a data processing system can solve the resulting system of equations for the x-ray absorbency of each voxel. The resulting data can then be analyzed or displayed as a three-dimensional model of the object that can be viewed from different viewpoints.
CT scanners are widely used for imaging the human body as part of diagnostic procedures. In principle and in limited practice, such scanners would be useful in imaging inanimate objects such as passenger baggage that is to be placed on an aircraft. Unfortunately, the cost of this equipment and its relatively low throughput has inhibited the use of CT scanners for such high volume applications. A baggage scanning system must process tens of bags per minute. Because they utilize fan-beam imaging and only obtain information from a single ‘slice’ or two-dimensional voxel set for each revolution of the imaging system, conventional CT scanners require tens of seconds to scan a patient's chest; hence, such systems fall short of the required throughput. In addition, many bags are much larger than the human chest; hence, the scan times using conventional CT scanners would be even longer. As a result of the high capital cost and low throughput, the cost per 3D scan using conventional CT scanners is prohibitive. In addition, the large footprint and low throughput of conventional CT scanners require the dedication of large amounts of floor space, which further increases the cost of utilizing such systems.
Broadly, it is the object of the present invention to provide an improved scanning x-ray inspection system for the generation of three-dimensional information.
This and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.